Anionic polymerization of conjugated dienes with lithium initiators, such as sec-butyllithium, and hydrogenation of residual unsaturation has been described in many references including U.S. Pat. No. Re. 27,145. The capping of living anionic polymers to form functional end groups is described in U.S. Pat. Nos. 4,417,029, 4,518,753, and 4,753,991. Of particular interest for the present invention are anionic polymers that are capped on one or more ends with hydroxyl, carboxyl, phenol, epoxy, and amine groups.
Anionic polymerization using protected functional initiators having the structure R.sup.1 R.sup.2 R.sup.3 Si--O--A'--Li is described in WO 91/12277 wherein R.sup.1, R.sup.2, and R.sup.3 are preferably alkyl, alkoxy, aryl, or alkaryl groups having from 1 to 10 carbon atoms, and A' is preferably a branched or straight chain bridging group having at least 2 carbon atoms. The bridging group is most preferably straight chain alkyl having from 3 to 10 carbon atoms and is exemplified by the following compound: ##STR1## which is readily prepared by lithiation of the reaction product of 1-chloro-6-hydroxy-n-hexane and t-butyldimethylchlorosilane.
Monofunctional and telechelic polymers produced by anionic polymerization of dienes and vinyl aromatic monomers typically have narrow molecular weight distributions in comparison to the broad molecular weight distributions usually achieved by polymerization of these monomers by non-anionic mechanisms such as free radical, cationic, Ziegler-Natta, etc. Number average molecular weights are preferred for the comparison of most functionally terminated polymers because they are reacted in stoichiometric amounts with crosslinking and chain extension agents such as poly- and di-functional isocyanates. The absence of very high molecular weight components in polymers produced by anionic polymerization results in low viscosities for a given number average molecular weight. Low viscosities are desirable in functionally terminated polymers for applications such as paints and coatings because they allow formulation with a minimum amount of solvent and/or elevated temperature to reach application viscosities.
Amorphous, low glass transition temperature polymers such as polydienes are advantageous for coatings applications because of their low viscosities. Hydrogenated dienes are particularly advantaged for applications that require good weatherability and hydrolytic stability. Hydrogenated isoprene is advantaged over hydrogenated polybutadiene because it does not crystallize and is, therefore, transparent. Hydrogenated isoprene polymers have lower viscosities than high 1,4-addition hydrogenated polybutadiene diols such as POLYTAIL H made by Mitsubishi which is a solid at room temperature. Although low 1,4-addition polybutadienes such as G-2000 made by Nisso or Polytail-HA made by Mitsubishi are transparent, they have high glass transition temperatures and high viscosities.
It is also desirable to avoid the presence of molecules with more than two functional groups to avoid crosslinking in applications such as thermoplastic polyurethanes and modification of polycarbonates and polyesters. Anionic polymerization using a protected functional initiator followed by end capping assures that no molecules have functionality greater than two, unlike radical polymerizations which have broad distributions of functionality.
Telechelic hydrogenated isoprene polymers having number average molecular weights from 2,500 to 5,500 are commercially available from Atochem under the name EPOL and Kuraray, TH-21 and TH-1, but all known commercial products have polydispersities (M.sub.w /M.sub.n or Q) greater than 2, viscosities higher than 500 poise at 25.degree. C., and average functionalities greater than 2.0. It is an object of the present invention to provide improved monofunctional and telechelic unsaturated and hydrogenated isoprene polymers having low viscosity.